ISOKINETIC MUSCLE STRENGTH IN NORMAL ADULTS: REVISITED

Abstract

PURPOSE In evaluating patients it is necessary to have understanding of the normal population. Although many authors have reported isokinetic normal strength values, several questions regarding testing protocol and data interpretation remain unanswered. This study was conducted to address the difference between preferred and stronger sides, determine a clinically relevant muscle imbalance threshold for various muscle groups, and to study the correlation of strength between various muscle groups. METHODS Fifteen subjects (8 female, 7 male) were evaluated. Average age was 29.9 (24 - 43 years). Isokinetic muscle strength was measured for Hip ext/flex, Hip abd/add, and Ankle pf/df at 30,60, and 120 degree/sec. Knee ext/ flex was collected at 60, 120 and 180 degree/sec. Data were collected on a Cybex II and analyzed using the CSMI-Humac 600. One evaluator encouraged subjects to familiarize themselves with the machine by a warm up session and then give a maximal effort for each movement, and not to stop until instructed to do so. If the repetitions were not within 15% of one another, the test was repeated for validity. Limb preference was recorded. Peak torque/body weight (PEAK) for all speeds, average of the 3 maximal repetitions1 body weight (AVG) for slow and medium speeds, and fatigue index for the high speed were computed for all speeds and all motions. Preferred vs stronger and stronger vs weaker comparisons were made using a paired t-test. Pearson correlations for all motions were computed. RESULTS - Limb preference is not a good indicator of stronger side in isokinetic muscle strength testing. Strongest to preferred side was compared in all variables by computing the percentage of reversals (stronger side was not the preferred side). For most joints 50 - 60% of the time the strongest side was not the preferred side, with the exception on ankle pf at 39%. Looking at all speeds and motions, the greatest incidence of reversals were in PEAK with 60% at high speed, 54% at medium, and 5 1 % at slow. The AVG was comparable with 5 1 % at medium, 58% at slow speeds and fatigue index was the lowest at 36%. Significant muscle imbalance, the percentage difference in PEAK from side to side, was noted. Muscle imbalance varied between sexes for hip ext (9%M vs 14%F), ankle pf (9%M vs 18%F) and ankle df (1 l%M vs 208F). The mean difference in PEAK was 13% for hip flex, 19% for hip abdladd, and 11% for knee flexlext. Analysis was done to determine whether particular muscle groups correlated in strength. All opposing muscle groups (hip ext 1 flex, knee ext / flex, hip flex knee flex and knee flex / ankle pf) were positively correlated (R = .8, pc.001). CONCLUSIONS Approximately 50% of the time, the strongest side was not the preferred side. Ankle pf, primary power genator during running and gait, had the lowest number of reversals. Clinical significant imbalance in muscle strength thresholds have previously been reported to be a difference > 20% is probably abnormal, 10-20% possibly abnormal, and up to 10% normal (Sapaga, 1990). In contrast, our results indicate that a greater imbalance (20%) can be expected for hip abd/add in the normal population. It was noted imbalance was greater in females as compared to males. A side to side difference of 20% in ankle pf/df is normal for females. There was a strong correlation between opposing muscle groups at all speeds. REFERENCES Sapaga, A.A. 1990 "Muscle performance evaluation in orthopaedic practice", Journal of Bone and Joint Surgery 72A:1562-1574.